Objective for an endoscope

ABSTRACT

An objective for an endoscope comprising a GRIN lens whose surface on the object side is arranged as a planar surface or convex surface and whose surface on the image side is arranged as a convex surface, the objective for an endoscope having an extremely small outer diameter and a wide field angle.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an objective for an endoscope and, moreparticularly, to an objective for an endoscope wherein an inhomogeneousmedium is used.

(b) Description of the Prior Art

A retrofocus type objective as shown in FIG. 1 and disclosed by Japanesepublished unexamined patent application No. 121547/74 is known as anobjective for an endoscope.

In recent years, it is required to make the endoscopes smaller andsmaller in the diameter. This means that an objective with an extremelysmall outer diameter is required necessarily.

For an objective with an extremely small outer diameter, it is verydifficult to use a plural number of lenses as in case of said knownretrofocus type objective due to the reasons related to assembly.Therefore, it is known to arrange that an objective comprises onepositive lens as shown in FIG. 2. However, an objective comprising onepositive lens has a disadvantage that aberrations are caused largelyand, consequently, the field angle is somewhat limited and cannot bemade wide.

To obtain an objective with a small outer diameter which supersedes saidknown objective comprising one positive lens, it may be considered touse a lens made of an inhomogeneous medium as disclosed by Japanesepublished examined patent application No. 28061/72 and as shown in FIG.3. For example, it may be considered to use a graded refractive indexlens (hereinafter referred to as a GRIN lens) whose refractive index isexpressed by the formula shown below when the refractive index of thecentral portion of said GRIN lens is represented by reference symbol n₀and the radial distance from the optical axis is represented byreference symbol r:

    n.sup.2 (r)=n.sub.0.sup.2 {1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 + . . . }

where, reference symbol g represents a parameter showing the degree ofthe gradient of the refractive index, and reference symbols h₄, h₆ . . .respectively represent the coefficients of distribution of refractiveindices in the terms of the fourth order, sixth order and so on.

However, said GRIN lens has a disadvantage that the field angle isdecided by the distribution of refractive indices thereof, and it isimpossible to make the field angle wide.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to providean objective for an endoscope having a simple composition, an extremelysmall outer diameter and a wide field angle.

The basic composition of the objective for an endoscope according to thepresent invention is shown in FIG. 4.

To attain the above-mentioned primary object, the objective for anendoscope according to the present invention is composed as describedbelow.

That is, the objective for an endoscope according to the presentinvention is arranged to comprise a GRIN lens with an extremely smallouter diameter and an aperture stop located on the object side of saidGRIN lens, said GRIN lens being arranged that the surface on the objectside is formed as a planar surface or convex surface and the surface onthe image side is formed as a convex surface and that the refractiveindex n of said GRIN lens is expressed by the formula shown below whenthe refractive index of the central portion of said GRIN lens isrepresented by reference symbol n₀ and the radial distance from theoptical axis is represented by reference symbol r:

    n.sup.2 (r)=n.sub.0.sup.2 {1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 + . . . }

where, reference symbol g represents a parameter showing the degree ofthe gradient of the refractive index, and reference symbols h₄, h₆, . .. respectively represent the coefficients of distribution of refractiveindices in the terms of the fourth order, sixth order and so on, saidobjective for an endoscope according to the present invention beingarranged to fulfill the conditions (1), (2) and (3) shown below:##EQU1## where, reference symbol I represents the image height,reference symbol φ represents the outer diameter of the GRIN lens,reference symbol Z represents the thickness of the central portion ofthe GRIN lens, reference symbol R₁ represents the radius of curvature ofthe surface on the object side of the GRIN lens, and reference symbol R₂represents the radius of curvature of the surface on the image side ofthe GRIN lens.

By arranging at least one surface of the GRIN lens as a convex surface,the objective for an endoscope according to the present invention isarranged to eliminate the disadvantage of the known objective for anendoscope comprising one positive lens as shown in FIG. 2 and of theknown objective for an endoscope comprising a GRIN lens as shown in FIG.3, i.e., the disadvantage that the field angle cannot be made wide.

In other words, the objective for an endoscope according to the presentinvention is arranged that at least one surface of the GRIN lens, whosemedium itself has power, is formed as a convex surface in the range thatoffaxial aberrations to be caused by said GRIN lens, i.e., coma andcurvature of field, are not undercorrected, said objective for anendoscope being thereby arranged that the power of the lens (the GRINlens) as a whole is made strong so that the field angle is thereby madewide.

As the outer diameter of the endoscope should be made small, theobjective constituting the endoscope should be arranged to comprise asingle lens whose outer diameter is extremely small. To make the fieldangle wide and to make the outer diameter of the endoscope small at thesame time, it is necessary to arrange that the image height I and outerdiameter φ of the single lens fulfill the condition (1) shown before. Ifthe value of |2I/φ| becomes smaller than the lower limit of thecondition (1), it is difficult to make the field angle wide. If thevalue of |2I/φ| becomes larger than the upper limit of the condition(1), it is difficult to make the outer diameter of the endoscope small.

In case of the objective according to the present invention, the imageheight, i.e., the field angle, is extremely large compared withobjectives for pick-up devices for optical disks and the like.Therefore, the objective according to the present invention is arrangedto fulfill the conditions (2) and (3) in order to make the field anglewide in the range that offaxial aberrations are not undercorrected.

The condition (2) defines the value obtained when the power (1-n₀)/R₂ ofthe surface on the image side of the single lens is divided by the powern₀ ·g·sin(gZ) of the medium itself which has the power due to the factthat the medium itself has the distribution of refractive indicestherein. In other words, the condition (2) defines the ratio between theformer power and the latter power.

If said ratio becomes smaller than the lower limit of the condition (2),the power of the surface on the image side of the single lens becomestoo weak, and it is difficult to make the field angle wider than thevalue decided by the distribution of refractive indices. If said ratiobecomes larger than the upper limit of the condition (2), the power ofthe surface on the image side of the single lens becomes too strong, andaberrations will be undercorrected.

The condition (3) defines the value obtained when the radius ofcurvature R₂ of the surface on the image side of the single lens isdivided by the radius of curvature R₁ of the surface on the object sideof said single lens. If the value of |R₂ /R₁ | becomes larger than 1.0defined by the condition (3), i.e., if R₁ becomes smaller than R₂, thepower of the surface on the object side becomes too strong, and offaxialaberrations will be undercorrected.

In addition to the conditions described so far, it is preferable toarrange that the objective for an endoscope according to the presentinvention further fulfills the conditions (4), (5) and (6) shown belowbecause it is then possible to obtain a more favourable objective for anendoscope which serves to attain the object of the present invention.

    |R.sub.2 /I|≦3                    (4)

    g≧0.58                                              (5)

    h.sub.4 ≧0                                          (6)

Out of respective conditions shown in the above, the conditions (4) and(5) respectively serve to make the power of the single lens, whose outerdiameter is extremely small, strong and to thereby make the field anglewide.

When, either the condition (4) or the condition (5) is not fulfilled, itis difficult to make the field angle wide.

The condition (6) relates to the coefficient h₄ of distribution ofrefractive indices in the term of the fourth order. When it is arrangedthat the value of h₄ becomes positive as defined by the condition (6),the gradient of distribution of refractive indices in the marginalportion of the medium becomes comparatively gentle, and the power of themarginal portion of the medium becomes weak. Therefore, it is possibleto correct offaxial aberrations to be caused by the convex surface ofthe single lens. On the contrary, when the value of h₄ becomes negative,offaxial aberrations will be undercorrected.

As shown in FIGS. 4 and 5, an image fiber bundle 2 may be generallyconsidered as a means for optically transmitting the image formed by thesingle lens 1 constituting the objective according to the presentinvention. In that case, it is preferable to arrange that the principalray 3 enters the light entrance surface of the image fiber bundle 2substantially vertically. For this purpose, the aperture stop S shouldbe provided on the object side of the single lens.

The case is the same as above also when a rod-like GRIN lens 4 as shownin FIG. 6 or a lens system 5 as shown in FIG. 7 is provided, instead ofthe image fiber bundle 2, as the means for optically transmitting theimage.

On the other hand, in case of an electronic endoscope wherein asolid-state image sensor 6 is used as the means for transmitting theimage as shown in FIG. 8, the image will be properly transmitted evenwhen the principal ray 3' is obliquely incident on the solid-state imagesensor 6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 3 respectively show sectional views illustratingcompositions of known objectives for endoscopes;

FIG. 4 shows a sectional view illustrating the basic composition of theobjective for an endoscope according to the present invention;

FIG. 5 shows a sectional view illustrating the composition of an examplewherein the objective according to the present invention is applied toan endoscope in which an image fiber bundle is used;

FIG. 6 shows a sectional view illustrating the composition of an examplewherein the objective according to the present invention is applied toan endoscope in which a rod-like GRIN lens is used as an imagetransmission system;

FIG. 7 shows a sectional view illustrating the composition of an examplewherein the objective according to the present invention is applied toan endoscope which employs an image transmission optical systemcomprising homogeneous lenses;

FIG. 8 shows a sectional view illustrating the composition of an examplewherein the objective according to the present invention is applied toan electronic endoscope which employs a solid-state image sensor;

FIG. 9 shows a sectional view of Embodiment 1 of the present invention;

FIG. 10 shows a sectional view of Embodiments 2, 3, 4, 5 and 7 of thepresent invention;

FIG. 11 shows a sectional view of Embodiments 6 and 8 of the presentinvention;

FIG. 12 shows a sectional view of Embodiment 9 of the present invention;

FIG. 13 shows a sectional view of Embodiments 10, 11, 12, 13 and 14 ofthe present invention;

FIG. 14 shows a sectional view of Embodiments 15, 16 and 18 of thepresent invention;

FIG. 15 shows a sectional view of Embodiment 17 of the presentinvention; and

FIGS. 16 through 33 respectively show graphs illustrating aberrationcurves of Embodiments 1 through 18 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the preferred embodiments of the objective for an endoscopeaccording to the present invention are shown below.

    ______________________________________                                        Embodiment 1                                                                  f = 0.491,    F = 2.99,    2ω = 69.2°                            I = 0.3,      φ= 0.6                                                      ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.6632                                                                              n.sub.1 = 1.6000 (*)                                          r.sub.3 = -0.4500 ( = R.sub.2)                                                d.sub.3 = 0.1900                                                              r.sub.4 = ∞                                                             d.sub.4 = 0.3000                                                                              n.sub.2 = 1.51633                                                                           ν.sub.2 = 64.15                              r.sub.5 = ∞                                                             ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR1##                                                                                   ##STR2##                                                         ##STR3##                                                                                   ##STR4##                                                        Embodiment 2                                                                  f = 0.551,    F = 2.99,    2ω = 60.8°                            I = 0.3,      φ= 0.6                                                      ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.7363                                                                              n.sub.1 = 1.6000 (*)                                          r.sub.3 = -0.6000 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR5##                                                                                   ##STR6##                                                         ##STR7##                                                                                   ##STR8##                                                        Embodiment 3                                                                  f = 0.589,    F = 2.99,    2ω = 56.6°                            I = 0.3       φ= 0.6                                                      ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.7868                                                                              n.sub.1 = 1.6000 (*)                                          r.sub.3 = -0.7500 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR9##                                                                                   ##STR10##                                                        ##STR11##                                                                                  ##STR12##                                                       Embodiment 4                                                                  f = 0.490,    F = 3.01,    2ω = 67.6°                            I = 0.3,      φ= 0.65                                                     ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.6774                                                                              n.sub.1 = 1.6000 (*)                                          r.sub.3 = -0.4500 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 =  0,     h.sub.6 = 0                                     ##STR13##                                                                                  ##STR14##                                                        ##STR15##                                                                                  ##STR16##                                                       Embodiment 5                                                                  f = 0.310,    F = 2.99,    2ω = 111.9°                           I = 0.3,      φ= 0.65                                                     ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.6238                                                                              n.sub.1 = 1.8000 (*)                                          r.sub.3 = -0.4500 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.5,     h.sub.4 = 2.5,    h.sub.6 = -0.5                                  ##STR17##                                                                                  ##STR18##                                                        ##STR19##                                                                                  ##STR20##                                                       Embodiment 6                                                                  f = 0.203,    F = 2.98     2ω = 83.9°                            I = 0.15,     φ= 0.35                                                     ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0     n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞  (stop)                                                     d.sub.2 = 0                                                                   r.sub.3 = ∞ (= R.sub.1 )                                                d.sub.3 = 0.3930                                                                              n.sub.2 = 1.65000 (*)                                         r.sub.4 = -0.2400 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 2.5,     h.sub.4 = 1.5,    h.sub.6 = 1                                     ##STR21##                                                                                  ##STR22##                                                        ##STR23##                                                                                  ##STR24##                                                       Embodiment 7                                                                  f = 0.219,    F = 2.99,    2ω =  77.8°                           I = 0.15,     φ= 0.35                                                     ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.3192                                                                              n.sub.1 = 1.65000 (*)                                         r.sub.3 = -0.2000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 2,       h.sub.4 = 4,      h.sub.6 = -1.5                                  ##STR25##                                                                                  ##STR26##                                                        ##STR27##                                                                                  ##STR28##                                                       Embodiment 8                                                                  f = 0.734,    F = 3.00,    2ω = 61.4°                            I = 0.4,      φ= 1.0                                                      ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.4000                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞ (stop)                                                      d.sub.2 = 0                                                                   r.sub.3 = ∞ (= R.sub.1 )                                                d.sub.3 = 0.9025                                                                              n.sub.2 = 1.60000 (*)                                         r.sub.4 = -0.6000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 0.61,    h.sub.4 = 2.2,    h.sub.6 = -30                                   ##STR29##                                                                                  ##STR30##                                                        ##STR31##                                                                                  ##STR32##                                                       Embodiment 9                                                                  f = 0.707,    F = 3.01,    2ω = 95.7°                            I = 0.6,      φ= 0.9                                                      ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.4000                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞(stop)                                                       d.sub.2 = 0                                                                   r.sub.3 = ∞ (= R.sub.1)                                                 d.sub.3 = 0.7888                                                                              n.sub.2 = 1.7000 (*)                                          r.sub.4 = -0.7000 (= R.sub.2)                                                 d.sub.4 = 0.4700                                                              r.sub.5 = ∞                                                             d.sub.5 = 0.3000                                                                              n.sub.3 = 1.51633                                                                           ν.sub.3 = 64.15                              r.sub.6 = ∞                                                             ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 0.65,    h.sub.4 = 2,      h.sub.6 = 10                                    ##STR33##                                                                                  ##STR34##                                                        ##STR35##                                                                                  ##STR36##                                                       Embodiment 10                                                                 f = 0.495,    F = 3.00,    2ω = 68.8°                            I = 0.3,      φ= 0.6                                                      ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 0.9000 (= R.sub.1)                                                  d.sub.2 = 0.7761                                                                              n.sub.1 =  1.6000 (*)                                         r.sub.3 = -0.6000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR37##                                                                                  ##STR38##                                                        ##STR39##                                                                                  ##STR40##                                                       Embodiment 11                                                                 f = 0.535,    F = 2.99,    2ω = 63.2°                            I = 0.3,      φ= 0.6                                                      ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 0.9000 (= R.sub.1)                                                  d.sub.2 = 0.8794                                                                              n.sub.1 = 1.6000 (*)                                          r.sub.3 = -0.9000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0      h.sub.4 = 0,      h.sub.6 = 0                                     ##STR41##                                                                                  ##STR42##                                                        ##STR43##                                                                                  ##STR44##                                                       Embodiment 12                                                                 f = 0.475,    F = 2.99,    2ω = 70.1°                            I = 0.3,      φ= 0.65                                                     ______________________________________                                        r.sub.1 =  ∞ (stop)                                                     d.sub.1 = 0                                                                   r.sub.2 = 1.0000 (= R.sub.1)                                                  d.sub.2 = 0.7435                                                                              n.sub.1 = 1.60000 (*)                                         r.sub.3 = -0.5000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 3,      h.sub.6 = 0                                     ##STR45##                                                                                  ##STR46##                                                        ##STR47##                                                                                  ##STR48##                                                       Embodiment 13                                                                 f = 0.507,    F = 3.01,    2ω = 65.0°                            I = 0.3,      φ= 0.65                                                     ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 0.7000 (= R.sub.1)                                                  d.sub.2 = 0.8874                                                                              n.sub.1 = 1.60000 (*)                                         r.sub.3 = -0.7000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 3,      h.sub.6 = 0                                     ##STR49##                                                                                  ##STR50##                                                        ##STR51##                                                                                  ##STR52##                                                       Embodiment 14                                                                 f = 0.317,    F = 3.00,    2ω = 108.9°                           I = 0.3,      φ= 0.65                                                     ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 1.0000 (= R.sub.1)                                                  d.sub.2 = 0.6798                                                                              n.sub.1 = 1.80000 (*)                                         r.sub.3 = -0.5000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.5,     h.sub.4 = 2.5,    h.sub.6 = -0.5                                  ##STR53##                                                                                  ##STR54##                                                        ##STR55##                                                                                  ##STR56##                                                       Embodiment 15                                                                 f = 0.201,    F = 2.99     2ω= 87.8°                             I = 0.15,     φ= 0.35                                                     ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.1500                                                                              n.sub.1 = 1.51633,                                                                          ν.sub.1 = 64.15                              r.sub.2 = ∞                                                             d.sub.2 = 0                                                                   r.sub.3 = ∞ (stop)                                                      d.sub.3 = 0.0500                                                              r.sub.4 = 0.4000 (= R.sub.1)                                                  d.sub.4 = 0.2429                                                                              n.sub.2 = 1.6500 (*)                                          r.sub.5 = -0.3000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 2.5,     h.sub.4 = 1.5,    h.sub.6 = 1                                     ##STR57##                                                                                  ##STR58##                                                        ##STR59##                                                                                  ##STR60##                                                       Embodiment 16                                                                 f = 0.236,    F = 3.04,    2ω = 71.5°                            I = 0.15,     φ= 0.35                                                     ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.500 n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞                                                             d.sub.2 = 0                                                                   r.sub.3 = ∞ (stop)                                                      d.sub.3 = 0.0500                                                              r.sub.4 = 0.4500 (= R.sub.1)                                                  d.sub.4 = 0.3785                                                                              n.sub.2 = 1.65000 (*)                                         r.sub.5 = -0.3000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 2,       h.sub.4 = 4,      h.sub.6 = -1.5                                  ##STR61##                                                                                  ##STR62##                                                        ##STR63##                                                                                  ##STR64##                                                       Embodiment 17                                                                 f = 0.830,    F = 3.00,    2ω = 80.6°                            I = 0.6,      φ= 1.0                                                      ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.4000 n.sub.1 = 1.51633                                                            ν.sub.1 = 64.15                                            r.sub.2 = ∞                                                             d.sub.2 =  0                                                                  r.sub.3 = ∞ (stop)                                                      d.sub.3 = 0.1000                                                              r.sub.4 = 2.5000 (= R.sub.1)                                                  d.sub.4 = 0.5850                                                                              n.sub.2 = 1.60000 (*)                                         r.sub.5 = -0.8000 (= R.sub.2)                                                 d.sub.5 = 0.3900                                                              r.sub.6 = ∞                                                             d.sub.6 = 0.6000                                                                              n.sub.3 = 1.51633                                                                           ν.sub.3 = 64.15                              r.sub.7 = ∞                                                             ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 0.61,    h.sub.4 = 2.2,    h.sub.6 = -30                                   ##STR65##                                                                                  ##STR66##                                                        ##STR67##                                                                                  ##STR68##                                                       Embodiment 18                                                                 f = 0.625,    F = 3.00,    2ω = 76.3°                            I = 0.4,      φ= 0.9                                                      ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.4000                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞                                                             d.sub.2 = 0                                                                   r.sub.3 = ∞ (stop)                                                      d.sub.3 = 0.1000                                                              r.sub.4 = 0.14000 (= R.sub.1)                                                 d.sub.4 = 0.9581                                                                              n.sub.2 = 1.70000 (*)                                         r.sub.5 = -0.7000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 0.65,    h.sub.4 = 2,      h.sub.6 = 10                                    ##STR69##                                                                                  ##STR70##                                                        ##STR71##                                                                                  ##STR72##                                                       ______________________________________                                    

In respective embodiments shown in the above, reference symbols r₁, r₂,. . . respectively represent radii of curvature of respective surfaces,in the order from the object side, including the stop and cover glass,reference symbols d₁, d₂, . . . respectively represent distances betweenrespective surfaces in the order from the object side, reference symbolsn₁, n₂, . . . respectively represent refractive indices of the lens, andreference symbol ν₁ and ν₂ represent Abbe's number of the lens made of ahomogeneous medium. In those embodiments wherein the stop is provided onthe lens surface or on the surface of the plane-parallel plate or thestop is provided at the cemented surface between the plane-parallelplate and GRIN lens, the stop and the surface, on which the stop isprovided, are shown separately and the distance between the stop andsaid surface is shown as 0. For the GRIN lens, the refractive index ofthe central portion thereof is shown and is marked with an asterisk (*).

Out of respective embodiments shown in the above, Embodiment 1 has thelens configuration as shown in FIG. 9. That is, Embodiment 1 comprises aGRIN lens 1, which is formed as a plano-convex single lens and arrangedthat a stop S is provided on the front surface thereof, and a coverglass 8.

Embodiments 2, 3, 4, 5 and 7 respectively have the lens configuration asshown in FIG. 10. That is, each of said embodiments comprises a GRINlens only which is formed as a plano-convex single lens and arrangedthat a stop S is provided on the front surface thereof.

Embodiments 6 and 8 respectively have the lens configuration as shown inFIG. 11. That is, each of said embodiments comprises a plane-parallelplate 9 made of a homogeneous medium and a GRIN lens 1 having aplano-convex shape which are cemented together, and a stop S is providedat the cemented surface between them.

Embodiment 9 has the lens configuration as shown in FIG. 12. That is,Embodiment 9 comprises a plane-parallel plate 9 made of a homogeneousmedium and a GRIN lens 1 having a plano-convex shape which are cementedtogether and arranged that a stop S is provided at the cemented surfacebetween them. Embodiment 9 further comprises a cover glass 8 arranged onthe image side of said GRIN lens 1.

Embodiments 10, 11, 12, 13 and 14 respectively have the lensconfiguration as shown in FIG. 13. That is, each of said embodimentscomprises a GRIN lens 1 having a biconvex shape and a stop S arranged infront of said GRIN lens 1.

Embodiments 15, 16 and 18 respectively have the lens configuration asshown in FIG. 14. That is, each of said embodiments comprises aplane-parallel plate 9, which is made of a homogeneous medium andarranged that a stop S is provided on the rear surface thereof, and aGRIN lens 1 having a biconvex shape.

Embodiment 17 has the lens configuration as shown in FIG. 15. That is,Embodiment 17 comprises a plane-parallel plate made of a homogeneousmedium and arranged that a stop S is provided on the rear surfacethereof, a GRIN lens 1 having a biconvex shape, and a cover glass 8.

Aberration curves of Embodiments 1 through 18 described so far are asshown in FIGS. 16 through 33 respectively.

What is claimed is:
 1. An objective for an endoscope comprising a GRINlens whose surface on the object side is arranged as a planar surface orconvex surface and whose surface on the image side is arranged as aconvex surface, and an aperture stop located on the object side of saidGRIN lens, said GRIN lens being arranged that the refractive index nthereof is expressed by the formula shown below when the refractiveindex of the central portion thereof is represented by reference symboln₀ and the radial distance from the optical axis is represented byreference symbol r, said objective for an endoscope fulfilling theconditions (1), (2) and (3) shown below:

    n.sup.2 (r)=n.sub.0.sup.2 {1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 + . . . } ##EQU2## where, reference symbol g represents a parameter showing the degree of the gradient of the refractive index, reference symbols h.sub.4, h.sub.6, . . . respectively represent the coefficients of distribution of refractive indices in the terms of the fourth order, sixth order and so on, reference symbol I represents the image height, reference symbol φ  represents the outer diameter of the GRIN lens, reference symbol Z represents the thickness of the central portion of the GRIN lens, and reference symbol R.sub.1 and R.sub.2 respectively represent radii of curvature of the surface on the object side and surface on the image side of the GRIN lens.


2. An objective for an endoscope according to claim 1 further fulfillingthe conditions (4), (5) and (6) shown below:

    |R.sub.2 /I|≦3                    (4)

    g≧0.58                                              (5)

    h.sub.4 ≧0                                          (6).


3. An objective for an endoscope according to claim 2 comprising saidGRIN lens and a cover glass wherein said GRIN lens is arranged to have aplano-convex shape and arranged that said aperture stop is provided onthe front surface of said GRIN lens, said objective for an endoscopehaving the following numerical data:

    ______________________________________                                        f = 0.491,    F = 2.99,    2ω = 69.2°                            I = 0.3,      φ = 0.6                                                     ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.6632                                                                              n.sub.1 = 1.6000 (*)                                          r.sub.3 = -0.4500 ( = R.sub.2)                                                d.sub.3 = 0.1900                                                              r.sub.4 = ∞                                                             d.sub.4 = 0.3000                                                                              n.sub.2 = 1.51633                                                                           ν = 64.15                                    r.sub.5 = ∞                                                             ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR73##                                                                                  ##STR74##                                                        ##STR75##                                                                                  ##STR76##                                                       ______________________________________                                    

where, reference symbols r₁, r₂, . . . respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop and cover glass, reference symbols d₁, d₂, . . .respectively represent distances between respective surfaces in theorder from the object side, reference symbols n₁, n₂, . . . respectivelyrepresent refractive indices of the lens, and reference symbol ν₂represents Abbe's number of the lens made of a homogeneous medium, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 4. An objective foran endoscope according to claim 2 comprising said GRIN lens wherein saidGRIN is arranged to have a plano-convex shape and arranged that saidaperture stop is provided on the front surface of said GRIN lens, saidobjective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.551,    F = 2.99,    2ω = 60.8°                            I = 0.3,      φ = 0.6                                                     ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.7363    n.sub.1 = 1.6000 (*)                                      r.sub.3 = -0.6000 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR77##                                                                                  ##STR78##                                                        ##STR79##                                                                                  ##STR80##                                                       ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order form the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 5. An objective foran endoscope according to claim 2 comprising said GRIN lens wherein saidGRIN lens is arranged to have a plano-convex shape and arranged thatsaid aperture stop is provided on the front surface of said GRIN lens,said objective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.589,    F = 2.99,    2ω = 56.6°                            I = 0.3,      φ  0.6                                                      ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.7868    n.sub.1 = 1.6000 (*)                                      r.sub.3 = -0.7500 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR81##                                                                                  ##STR82##                                                        ##STR83##                                                                                  ##STR84##                                                       ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁ and d₂ respectively representdistances beteen respective surfaces in the order from the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 6. An objective foran endoscope according to claim 2 comprising said GRIN lens wherein saidGRIN lens is arranged to have a plano-convex shape and arranged thatsaid aperture stop is provided on the front surface of said GRIN lens,said objective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.490,    F = 3.01,    2ω = 67.6°                            I = 0.3,      φ = 0.65                                                    ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.6774    n.sub.1 = 1.6000 (*)                                      r.sub.3 = -0.4500 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 3,      h.sub.6 = 0                                     ##STR85##                                                                                  ##STR86##                                                        ##STR87##                                                                                  ##STR88##                                                       ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order from the object side,and reference symbols n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 7. An objective foran endoscope according to claim 2 comprising said GRIN lens wherein saidGRIN lens is arranged to have a plano-convex shape and arranged thatsaid aperture stop is provided on the front surface of said GRIN lens,said objective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.310,    F = 2.99,    2ω = 111.9°                           I = 0.3,      φ = 0.65                                                    ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.2 = 0.6238    n.sub.1 = 1.8000 (*)                                      r.sub.3 = -0.4500 ( = R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.5,     h.sub.4 = 2.5,    h.sub.6 = -0.5                                  ##STR89##                                                                                  ##STR90##                                                        ##STR91##                                                                                  ##STR92##                                                       ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order from the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 8. An objective foran endoscope according to claim 2 comprising a plane-parallel plate madeof a homogeneous medium and said GRIN lens wherein said GRIN lens has aplano-convex shape, said plane-parallel plate and said GRIN lens arecemented together, and said aperture stop is provided at the cementedsurface between said plane-parallel plate and said GRIN lens, saidobjective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.203,    F = 2.98,    2ω = 83.9°                            I = 0.15,     φ= 0.35                                                     ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.1500 n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                             r.sub.2 = ∞ (stop)                                                      d.sub.2 = 0                                                                   r.sub.3 = ∞ (= R.sub.1 )                                                d.sub.3 = 0.3930 n.sub.2 = 1.65000 (*)                                        r.sub.2 = -0.2400 (=  R.sub.2)                                                ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 2.5,     h.sub.4 = 1.5,    h.sub.6 = 1                                     ##STR93##                                                                                  ##STR94##                                                        ##STR95##                                                                                  ##STR96##                                                       ______________________________________                                    

where, reference symbols r₁, r₂, . . . respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁, d₂, . . . respectivelyrepresent distances between respective surfaces in the order from theobject side, reference symbols n₁ and n₂ respectively representrefractive indices of the lens, and reference symbol ν₁ representsAbbe's number of the lens made of a homogeneous medium, and where, forthe GRIN lens, the refractive index of the central portion thereof isshown and is marked with an asterisk (*).
 9. An objective for anendoscope according to claim 2 comprising said GRIN lens wherein saidGRIN lens is arranged to have a plano-convex shape and arranged thatsaid aperture stop is provided on the front surface of said GRIN lens,said objective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.219,    F = 2.99,    2ω = 77.8°                            I = 0.15,     φ = 0.35                                                    ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = ∞ (= R.sub.1)                                                 d.sub.1 = 0.3192    n.sub.1 = 1.65000 (*)                                     r.sub.3 = -0.2000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 2,       h.sub.4 = 4,      h.sub.6 = -1.5                                  ##STR97##                                                                                  ##STR98##                                                        ##STR99##                                                                                  ##STR100##                                                      ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order from the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 10. An objectivefor an endoscope according to claim 2 comprising a plane-parallel platemade of a homogeneous medium and said GRIN lens wherein said GRIN lenshas a plano-convex shape, said plane-parallel plate and said GRIN lensare cemented together, and said aperture stop is provided at thecemented surface between said plane-parallel plate and said GRIN lens,said objective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.734,     F = 3.00,    2ω = 61.4°                           I = 0.4,       φ = 1.0                                                    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.4000                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞ (stop)                                                      d.sub.2 = 0                                                                   r.sub.3 = ∞ (= R.sub.1)                                                 d.sub.3 = 0.9025                                                                              n.sub.2 = 1.60000 (*)                                         r.sub.4 = -0.6000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 0.61,    h.sub.4 = 2.2,    h.sub.6 = -30                                   ##STR101##                                                                                 ##STR102##                                                       ##STR103##                                                                                 ##STR104##                                                      ______________________________________                                    

where, reference symbols r₁, r₂, . . . respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁, d₂, . . . respectivelyrepresent distances between respective surfaces in the order from theobject side, reference symbols n₁ and n₂ respectively representrefractive indices of the lens, and reference symbol ν₁ representsAbbe's number of the lens made of a homogeneous medium, and where, forthe GRIN lens, the refractive index of the central portion thereof isshown and is marked with an asterisk (*).
 11. An objective for anendoscope according to claim 2 comprising a plane-parallel plate made ofa homogeneous medium, said GRIN lens and a cover glass wherein said GRINlens has a plano-convex shape, said plane-parallel plate and said GRINlens are cemented together, and said aperture stop is provided at thecemented surface between said plane-parallel plate and said GRIN lens,said objective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.707,     F = 3.01,    2ω = 95.7°                           I = 0.6,       φ = 0.9                                                    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.4000                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞ (stop)                                                      d.sub.2 = 0                                                                   r.sub.3 = ∞ (= R.sub.1)                                                 d.sub.3 = 0.7888                                                                              n.sub.2 = 1.7000 (*)                                          r.sub.4 = -0.7000 (= R.sub.2)                                                 d.sub.4 = 0.4700                                                              r.sub.5 = ∞                                                             d.sub.5 = 0.3000                                                                              n.sub.3 = 1.51633                                                                           ν.sub.3 = 64.15                              r.sub.6 = ∞                                                             ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 0.65,    h.sub.4 = 2,      h.sub.6 = 10,                                   ##STR105##                                                                                 ##STR106##                                                       ##STR107##                                                                                 ##STR108##                                                      ______________________________________                                    

where, reference symbols r₁, r₂, . . . respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop and cover glass, reference symbols d₁, d₂, . . .respectively represent distances between respective surfaces in theorder from the object side, reference symbol n₁, n₂, . . . respectivelyrepresent refractive indices of the lens, and reference symbols ν₁ andν₃ respectively represent Abbe's numbers of lenses made of a homogeneousmedium, and where, for the GRIN lens, the refractive index of thecentral portion thereof is shown and is marked with an asterisk (*). 12.An objective for an endoscope according to claim 2 comprising said GRINlens, which is arranged to have a biconvex shape, and said aperture stoparranged in front of said GRIN lens, said objective for an endoscopehaving the following numerical data:

    ______________________________________                                        f = 0.495,     F = 3.00,    2ω = 68.8°                           I = 0.3,       φ = 0.6                                                    ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 0.9000 (= R.sub.1 )                                                 d.sub.2 = 0.7761     n.sub.1 = 1.6000 (*)                                     r.sub.3 = -0.6000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR109##                                                                                 ##STR110##                                                       ##STR111##                                                                                 ##STR112##                                                      ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order from the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 13. An objectivefor an endoscope according to claim 2 comprising said GRIN lens, whichis arranged to have a biconvex shape, and said aperture stop arranged infront of said GRIN lens, said objective for an endoscope having thefollowing numerical data:

    ______________________________________                                        f = 0.535,     F = 2.99,    2ω = 63.2°                           I = 0.3,       φ = 0.6                                                    ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 0.9000 (= R.sub.1 )                                                 d.sub.2 = 0.8794     n.sub.2 = 1.6000 (*)                                     r.sub.3 = -0.9000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 0,      h.sub.6 = 0                                     ##STR113##                                                                                 ##STR114##                                                       ##STR115##                                                                                 ##STR116##                                                      ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order from the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 14. An objectivefor an endoscope according to claim 2 comprising said GRIN lens, whichis arranged to have a biconvex shape, and said aperture stop arranged infront of said GRIN lens, said objective for an endoscope having thefollowing numerical data:

    ______________________________________                                        f = 0.475,     F = 2.99,    2ω = 70.1°                           I = 0.3,       φ = 0.65                                                   ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 1.0000 (= R.sub.1)                                                  d.sub.2 = 0.7435     n.sub.1 = 1.60000 (*)                                    r.sub.3 = -0.5000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 3,      h.sub.6 = 0                                     ##STR117##                                                                                 ##STR118##                                                       ##STR119##                                                                                 ##STR120##                                                      ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order from the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 15. An objectivefor an endoscope according to claim 2 comprising said GRIN lens, whichis arranged to have a biconvex shape, and said aperture stop arranged infront of said GRIN lens, said objective for an endoscope having thefollowing numerical data:

    ______________________________________                                        f = 0.507,     F = 3.01,    2ω = 65.0°                           I = 0.3,       φ = 0.65                                                   ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 0.7000 (= R.sub.1)                                                  d.sub.2 = 0.8774     n.sub.1 = 1.60000 (*)                                    r.sub.3 = -0.7000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.0,     h.sub.4 = 3,      h.sub.6 = 0                                     ##STR121##                                                                                 ##STR122##                                                       ##STR123##                                                                                 ##STR124##                                                      ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order from the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 16. An objectivefor an endoscope according to claim 2 comprising said GRIN lens, whichis arranged to have a biconvex shape, and said aperture stop arranged infront of said GRIN lens, said objective for an endoscope having thefollowing numerical data:

    ______________________________________                                        f = 0.317,    F = 3.00,    2ω = 108.9°                           I = 0.3,      φ = 0.65                                                    ______________________________________                                        r.sub.1 = ∞ (stop)                                                      d.sub.1 = 0                                                                   r.sub.2 = 1.0000 (= R.sub.1)                                                  d.sub.2 = 0.6798     n.sub.1 = 1.80000 (*)                                    r.sub.3 = -0.5000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 1.5,     h.sub.4 = 2.5,    h.sub.6 = -0.5                                  ##STR125##                                                                                 ##STR126##                                                       ##STR127##                                                                                 ##STR128##                                                      ______________________________________                                    

where, reference symbols r₁, r₂ and r₃ respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁ and d₂ respectively representdistances between respective surfaces in the order from the object side,and reference symbol n₁ represents refractive index of the lens, andwhere, for the GRIN lens, the refractive index of the central portionthereof is shown and is marked with an asterisk (*).
 17. An objectivefor an endoscope according to claim 2 comprising a plane-parallel plate,which is made of a homogeneous medium and arranged that said aperturestop is provided on the rear surface thereof, and said GRIN lens whichis arranged to have a biconvex shape, said objective for an endoscopehaving the following numerical data:

    ______________________________________                                        f = 0.201,     F = 2.99,    2ω = 87.8°                           I = 0.15,      φ = 0.35                                                   ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.1500                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞                                                             d.sub.2 = 0                                                                   r.sub.3 = ∞ (stop)                                                      d.sub.3 = 0.0500                                                              r.sub.4 = 0.4000 (= R.sub.1)                                                  d.sub.4 = 0.2429                                                                              n.sub.2 = 1.65000 (*)                                         r.sub.5 = -0.3000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 2.5,     h.sub.4 = 1.5,    h.sub.6 = 1                                     ##STR129##                                                                                 ##STR130##                                                       ##STR131##                                                                                 ##STR132##                                                      ______________________________________                                    

where, reference symbols r₁, r₂, . . . respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, referenc symbols d₁ and d₂, . . . respectivelyrepresent distances between respective surfaces in the order from theobject side, reference symbols n₁ and n₂ respectively representrefractive indices of the lens, and reference symbol ν₁ representsAbbe's number of the lens made of a homogeneous medium, and where, forthe GRIN lens, the refractive index of the central portion thereof isshown and is marked with an asterisk (*).
 18. An objective for anendoscope according to claim 2 comprising a plane-parallel plate, whichis made of a homogeneous medium and arranged that said aperture stop isprovided on the rear surface thereof, and said GRIN lens which isarranged to have a biconvex shape, said objective for an endoscopehaving the following numerical data:

    ______________________________________                                        f = 0.236,     F = 3.04,    2ω = 71.5°                           I = 0.15,      φ = 0.35                                                   ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.1500                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞                                                             d.sub.2 = 0                                                                   r.sub.3 = ∞ (stop)                                                      d.sub.3 = 0.0500                                                              r.sub.4 = 0.4500 (= R.sub.1)                                                  d.sub.4 = 0.3785                                                                              n.sub.2 = 1.65000 (*)                                         r.sub.5 = -0.3000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 2,       h.sub.4 = 4,      h.sub.6 = -1.5                                  ##STR133##                                                                                 ##STR134##                                                       ##STR135##                                                                                 ##STR136##                                                      ______________________________________                                    

where, reference symbols r₁, r₂, . . . respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁, d₂, . . . respectivelyrepresent distances between respective surfaces in the order from theobject side, reference symbols n₁ and n₂ respectively representrefractive indices of the lens, and reference symbol ν₁ representsAbbe's number of the lens made of a homogeneous medium, and where, forthe GRIN lens, the refractive index of the central portion thereof isshown and is marked with an asterisk (*).
 19. An objective for anendoscope according to claim 2 comprising a plane-parallel plate, whichis made of a homoheneous medium and arranged that said aperture stop isprovided on the rear surface thereof, said GRIN lens which is arrangedto have a biconvex shape, and a cover glass, said objective for anendoscope having the following numerical data:

    ______________________________________                                        f = 0.830,     F = 3.00,    2ω = 80.6°                           I = 0.6,       φ = 1.0                                                    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.4000                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞                                                             d.sub.2 = 0                                                                   r.sub.3 = ∞ (stop)                                                      d.sub.3 = 0.1000                                                              r.sub.4 = 2.5000 (= R.sub.1)                                                  d.sub.4 = 0.3785                                                                              n.sub.2 = 1.60000 (*)                                         r.sub.5 = -0.8000 (= R.sub.2)                                                 d.sub.5 = 0.3900                                                              r.sub.6 = ∞                                                             d.sub.6 = 0.6000                                                                              n.sub.3 = 1.51633                                                                           ν.sub.3 = 64.15                              r.sub.7 = ∞                                                             ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 0.61,    h.sub.4 = 2.2     h.sub.6 = -30                                   ##STR137##                                                                                 ##STR138##                                                       ##STR139##                                                                                 ##STR140##                                                      ______________________________________                                    

where, reference symbols r₁, r₂, . . . respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop and cover glass, reference symbols d₁, d₂, . . .respectively represent distances between respective surfaces in theorder from the object side, reference symbols n₁, n₁, . . . respectivelyrepresent refractive indices of the lens, and reference symbols ν₁ andν₃ respectively represent Abbe's number of the lens made of ahomogeneous medium, and where, for the GRIN lens, the refractive indexof the central portion thereof is shown and is marked with an asterisk(*).
 20. An objective for an endoscope according to claim 2 comprising aplane-parallel plate, which is made of a homogeneous medium and arrangedthat said aperture stop is provided on the rear surface thereof, andsaid GRIN lens which is arranged to have a biconvex shape, saidobjective for an endoscope having the following numerical data:

    ______________________________________                                        f = 0.625,     F = 3.00,    2ω = 76.3°                           I = 0.4,       φ = 0.9                                                    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 0.4000                                                                              n.sub.1 = 1.51633                                                                           ν.sub.1 = 64.15                              r.sub.2 = ∞                                                             d.sub.2 = 0                                                                   r.sub.3 = ∞ (stop)                                                      d.sub.3 = 0.1000                                                              r.sub.4 = 1.4000 (= R.sub.1)                                                  d.sub.4 = 0.9581                                                                              n.sub.2 = 1.70000 (*)                                         r.sub.5 = -0.7000 (= R.sub.2)                                                 ______________________________________                                        (Coefficients of power distribution of GRIN lens)                             g = 0.65,    h.sub.4 = 2,      h.sub.6 = 10                                    ##STR141##                                                                                 ##STR142##                                                       ##STR143##                                                                                 ##STR144##                                                      ______________________________________                                    

where, reference symbols r₁, r₂, . . . respectively represent radii ofcurvature of respective surfaces, in the order from the object side,including the stop, reference symbols d₁ and d₂, . . . respectivelyrepresent distances between respective surfaces in the order from theobject side, reference symbols n₁ and n₂ respectively representrefractive indices of the lens, and reference symbol ν₁ representsAbbe's number of the lens made of a homogeneous medium, and where, forthe GRIN lens, the refractive index of the central portion thereof isshown and is marked with an asterisk (*).